Electrical filter



June 3, 1941.

N. M. RUST ET AL ELECTRICAL FILTER Filed April 27, 1959 3 Sheets-Sheet l INVENTORS NOEL MEYER RUST ERA/EST F- GOODZVOUGH BY ATTORNEY June 3, 1941.

N. M. RUST ET AL ELECTRICAL FILTER Filed April 27, 1939 5 Sheets-Sheet 2 H v 6 Tu w Y owe E T N NR R 50 0 VYG T F m my EEY 5 22, MAX.

June 3, 1941.

N. M. RUST ET AL 2,244,022

ELECTRICAL FILTER Filed April 27, 1939 3 Sheets-Sheet 3 INVENTORS NOEL MEYER RUST ERNEST/ 0 ENOUGH ATTORNEY Patented June 3, 1941 ELECTRICAL FILTER N 0651 Meyer Rust, Danbury Common, Chelmsford, and Ernest Frederick Goodenough, Springfield,

Chelmsford, England, assignors to Radio Corporation of America, a corporation of Delaware Application April 2'7, 11939, Serial No. B70518 In Great Britain April 23, 11938 3 Claims.

This invention relates to electrical filters and more particularly to so-called band pass filters. The main object of the invention is to provide an improved easily manufactured and adjusted band pass filter which shall have a response curve with a flat top and steep sides, and in which the width of the top shall be capable of easy variation. Though not limited to its application thereto the invention is particularly suitable for represents the coupling between IFI and OFI.

In Figure 1 the crystal X is parallel with a variable condenser VC and the whole network couples the plate circuit of a valve Vi to the grid circuit use as a beat frequency, adjustable selectivity of a valve V2. band pass filter in a superheterodyne radio or In a modified form of the invention illustrated like receiver. in the accompanying Figure 2 the two mutually According to this invention a four-terminal coupledlfurtheor inductalnces IFI, OFI fof gigure 1 band pass filter network comprises a pair of are rep aced y a singe equivalen ur er ininput terminals and a pair of output terminals, ductance IOFI connected between the common one terminal of one pair being common with terminal and the corresponding ends of the main one terminal of the other; input and output inductances IMI, OMI. Further, if desired, and circuits connected respectively between the inas shown in the accompanying Figure 3, there put and between the output terminals and may be interposed in series with the single equivsaid input circuit including a tuning conalent further inductance IOFI or (as shown in denser connected between the input terminals the accompanying Figure 4) between the junc and in parallel with a main inductance in series tion point of the two mutually coupled further with a further inductance and said output circuit inductances IFI, OFI and the common terminal similarly including a tuning condenser connected (as the case may be) a resistance CR of desired between the output terminals and in parallel magnitude. The two points between which the with a main inductance in series with a further piezo electric crystal is connected may be, as inductance, the two further inductances being shown in Figures 1 to 4, intermediate tap points mutually coupled; and a piezo electric crystal on the two main inductances IMI and OMI or connected between a point on one main inductthey may be at those ends of said main inductn d a c sp n p in on he h r. so ances which correspond one with an input tern t e d aw Figs- 1 t0 4 e Various mOdiminal and the other with an output terminal. fications of electrical filter networks in accordt il b appreciated t in any event the ance with the invention. Figs.5 to 8 are response crystal will be shunted by the capacity of its curves obtained with appropriate adjustments of holder but t capacity may, if desired, be the filter networks disclosed in Figs. 1 to 4. Fig. 5 plemented by an actual condenser connected 9 is the electrical equivalent of the filter networks across the crystaL condenser VC is such a of e 1 to 10 1S furfiher eflulvalent denser. Where two mutually coupled further circuit of the filter networks. Fig. 11 is an exinductances are employed as in Figures 1 and 4 planatory vector dlagram the behavlour of i the mutual inductance is preferably arranged to filter netwoflzs at frequencles Close t be variable as indicated. Again, where a confrequency passed band 1S denser VC' is connected across the crystal, this further modification of a filter network in acmay be arranged to be Variable as Shown In dance with the invention. Figs. 13 to 15 are cor additlon, provision may be made for varying the alternative forms of couplings that may be used tumn of the t t d IT OT in place of that shown in Fig. 12. Fig. 1c is a g W0 unmg 9 ensers n e such manner that the capacity of one condenser still further modified form of band pass filter in which reaction is employed, and Figs. 17a and 1s f and that 9 the other decreasfad 171) are circuits which serve to explain the operaapproximately equally slmultaneously Vlce tion of the circuit of Fig 16. versa), preferably by gang control (as 1nd1cated one embodiment of the invention is Shown conventionally in Figures 1 to 4) of these condiagrammatically in the accompanying Figure l densers and/or by Varymg the 190511310115 0f the in which in and out are, respectively, the taps TI, TO, between which the crystal is coninput and output terminals; 10 and DC the input g if i If f t f gg glmbby tlglang control. t and out ut circuits; IT, IMI, and IFI the tuning e e co s o aina e y e various adju condens r, main inductance and further inductments are as follows:

of the sides, moving one dip closer. to the fiat top and the other further away therefrom, or vice versa, according to the adjustment. At the same time the skirt rises on either side of the dips become uneven, the greater rise corresponding to the dip which is closer. to. the.

mid-band frequency or flat top. Thisis represented in the accompanying Figure 6.

(3) Variation of the taps T1 T0. between which the crystal is connected, leaves. the. dip frequencie practically unaltered. but controls selectivity by varying the slopes of the sides. and controlling the width of the top. Thisis rep.- resented in the accompanying Figure 7. Variation of the tap will also involve. alteration of condenser VC or of the mutual inductance M.

(4 Variation of the tuning. of. the two tuning condensers IT, OT, produces a result similar to that produced. by (3) above and. represented in Figure 7.

(5) Simultaneous eifectingof variations asset forth in (l) and (3) above, or as set forth. in (1) and l) above, leaves the slopes of the sides practically unaltered but. controls the selectivity by controlling the width of the top and simultaneously moving the dips towards or away from one another. t the same time the rises outwardly of the dips are varied, the said rises becoming greater as the selectivity is :increased. This is represented in the accompanying Figure 8.

The functioning of a filter in accordance with this invention may best be described by considering the equivalent electrical circuit of a filter as above described. Thisequivalent circuit is represented in the accompanying Figure 9- and comprises'an impedance Z! constituted by a parallel circuit consisting of inductance L and parallelcapacity. C connected between-theinput terminals; a similarly constituted parallel circuit impedance Zl between the output terminals; 9. seriesresonant circuit comprising capacity Ceresistanoe re and inductance Le all in series (this is'the equivalent of the crystal) between one input terminal and one-output terminal; a capacityCs connected across said series resonant circuit (this represents the crystalholder capacity plus any supplementary capacity); and an indluctance L5 in series with a resistance Ts connected-across the capacity Cs. The series connected elements Ls and Ts constitute the delta equivalent of the star network M, R, and Li-M (for the case where mutual inductance M- is employed between coupled further inductances, L1 being the inductance of each main inductance IMI, OM-I and R the resistance (CR) in: series with the further inductances IFI, OFE), or thedelta equivalent. of the star network LcRLz (for the case where a single equivalent further inductance IOFI is employed L/3 is that inductance, R the resistance (CR) in series therewith and-L2 the inductance of each maininductance IMI,

OMI). In the one case (the mutual inductance case) L =Qi K 1 and and in the other L L L 2 In these formulae. n is the inherent resistance L1 or L2 asthe case may be.

Thus the action of M or M3 is'in effect to place aninductance L5 across the capacity CS; this maybe adjusted to resonate with Cs at the same frequency as that at which the crystal resonates. Moreover by adjustment of the coupling resistance R, the resistance T5 in series With-Ls may be Ina-dezero or negative, so that the circuit Cs Ls is in effect loss-less. This shunts the crystal which as stated above is equivalent to a series resonant circuit, but whose component values moreover are vastly different from anything obtainable by ordinary circuits. For instance, a crystal suitable for this typeof filter and cut to oscillate at 450 kc. has equivalent component values Lc=2.84 henries, Cc=.0433 mmfd., r=l,0()0 ohms, and is thus a high Q circuit as well as of relatively high impedance.

Let the input and output circuits-hereinafter termed the side circuits-be designated-simply Z1" and the circuit between one input terminal and'one output terminal Z2 (see- Figure 9 Z2 may be-regarded as represented in the accompanying Figure 10-as consisting of two parallel tuned low-loss circuits in series, one tuned alittle above and the other a little below the mid-band frequency In, and presenting very high impedances at their resonance points at which, accordingly, the filter output will be very small, i. e. the dips in the response curve will occur at these resonant frequencies. In the neighborhood of f0, however, Z2 becomes, in effect, a series resonant" circuit presenting only the resistance of the crystal, which is small compared to zi. It may be shown, that the output of the filter is proportional' to and 1+ z') Ata. frequency slightly above f0, Z1 (-tunedto f0) becomes capacitative and Z2 inductive, and these reactances' tendv to cancel. Owing, tothis fact the magnitude of ZZI-{TZZ tends to: decrease with frequency change at approximately the-same rate as Z1 with the result that aslong as; theabove condition obtains,,the output remainspractically constant for a. given. current input. A. decrease from. ft. in the frequency of the applied current causes Z1 to becomeinductive. and;Zz; capacitative with a similar result. As the dip frequency is approached, the response of course drops.

If the tuning of the input and output: circuits Z1 is changed, one being tuned: to a frequency slightly abovethe mid band frequency f0, and'the other slightly below (i. e. their tuning is staggered), their added impedance is reduced and the controlling effect of the crystal is increased. Moreover, their added reactance at frequencies near f is reduced to a still greater extent, so that it no longer cancels the crystal reactance. The combined efiect is to sharpen the response curve to an extent depending on the amount of stagger. 1

The output of the filter at In is not afiected by staggering. 7

By tuning Ls C to a slightly different frequency, e. g. by varying M or Cs the resonant frequencies of the series connected parallel tuned circuits regarded as comprised in Z2 and therefore the positions of the dips may be altered asymmetrically with respect to in. One side of the response curve may be steepened considerably by this means.

A more rigorous examination of the circuit will now be made to find the relationships between the various component values, the separation of the dip frequencies and the width of the pass band.

Consider the case where the filter is used in an I. F. amplifier, for example, and is connected between the anode of a high impedance valve (screened grid or pentode) and the grid of another valve (see Figures 1 to 4) The anode impedance of the valve connected to the input may be assumed highcompared with the input impedance of the filter so that the input current is practically constant over the frequency range considered for a constant voltage applied to the grid of that valve.

The voltage E at the output terminals of a three element filter such as Z1, Z2, Z1, such as that of Figure 10, is given by Since the input current i is constant in this case the output voltage is proportional to To find the value of Z2 in terms of the crystal constants and its shunt capacity Cs first find its admittance by adding the admittance of the circuits comprising it. Since both are of high Q their resistances may be neglected at frequencies slightly removed from f0.

The admittance of the parallel resonant cir cuit Ls C5 is 1 where Put to m :lzdw

where dw m Then

and

co ca i d w since and is positive for an increase in frequencyand negative for a decrease.

Z2 is therefore greatest when 1 (211m) L C, l i. e. when :l: 1 /L,c. where fin is the frequency separation of the clips and is incidentally the resonance frequency of Lc and C5.

The behaviour of the network at frequencies close to f0 may be visualized by means of the vector diagrams of Z1 and Z2.

From the expression for Z2 above, it is clear that its vector diagram will be as represented in the accompanying Figure 11 a straight line passing practically through the origin (since To is small). The vectors move upwards from 0 for small positive values of liar and downwards for negative values, their lengths increasing uniformly at first, and then increasingat a greater rate as (2dw) LcCs approaches unity. Eventually they change sign and decrease in length.

Half the vector diagram for 221 will be a semicircle ()BB (see Figure 11) on a diameter at right angles to the above mentioned straight line vector diagram for Z2. This diameter is drawn equivalent to the magnitude of 221 at resonanceto the same scale as that adopted for Z2.

It is convenient to reverse one of the vectors (that for Z2 say) and the sum 2Z1+Z2 will then be seen to be represented by the line AB joining the ends of the vectors 0A and OB for Z2 and 2Z1 for each particular frequency.

To obtain an expression for Z1 in terms of (in! add T-l-j'wL and w LC The magnitude of ZlriS therefore lmax viii? and its real part is Hence Z Z X (the real part of Z) l 1 +192 1mm! Therefore Z1 'is proportional to the length of the line CB. Hence the output of the filter is proportional to the ratio of the length CB to the length AB. This ratio is equal to unity at in corresponding to maximum output, and also at the frequency for which OA=OC as at A and B. Between this frequency and ,fo the ratio CB/AB is slightly lessthan unity but at greater frequencies A lengthens rapidly so-that the said ratio decreases rapidly and a sharp cut off is obtained. At still greater frequencies the ratio increases again somewhat, giving a rising skirt. The action is similar on both sides of in giving a practically symmetrical pass band between cutoff frequencies outwardly of which are rising skirts.

The frequencies at which CB=AB may be found by equating Z2 and the reactive component of ZZBi thus:

Hence substituting for ZmaxL/r and cancelling 211w,

L. 2LQ2 2L o m 1 my 1 e Q Q2 e e an 2a are a e wo fo o 8 where fb1=the peak separation or pass band width L=L1+M or-Lz+2Ls (as the case may be).

Then

'T f0 7 2 Iii) c fb? which, re-arranged, gives In) L ale +02 ILC"" l-fl 2 From these equations the values of Lo and Cs 7 may be determined in terms of L, Q, fin, and fbz.

The following is an example of practical figures employed in an experimentally tested filter in accordance with this invention:

L1=1,340 micro-'henries Lc=3.08 henries Cs=10 micro-microfarads Q :50

in :500 kilocycles.

In this filter there was a substantially flat top. between ikc. from f0, the-output falling to about65 db, (taking the top as zero) at cutoff frequencies at about :13 kc. from in, the skirts rising to about -4=0 db. at about :25 kc. from in.

In the filters so far described, as positive mutual inductance or equivalent self-inductance is used to couple two tuned circuits which are also coupled by a crystal; the inductive coupling is intended to counteract coupling due to unavoidable shunt capacity across the crystal. As has been shown above, the small shunt mutual inductanceis equivalent to a large inductance (Ls) shunting the crystal and which may be made to tune with the capacity (Cs) also shunting the crystal. A defect is that Ls and Cs must be tuned very accurately to the resonancefrequency of the crystal, and this entails careful adjustment of either the mutual coupling or of Cs. Alteration of the mutual coupling, of course, alters the tuning of the side circuits so that the tuning conden-sers have to be readjusted and variation of Cs is often impracticable because it has to be kept as small as possible and the provision of an extra capacity (such as is shown in the preceding figure) for tuning purposes may not be permissible. Again C5 is at the high A. C. potential position in the filter and it may, therefore, be required to be heavily screened making adjustment physically impracticable. Further, variation of the mutual inductance involves unwieldy and relatively expensive apparatus.

In the embodiments now to be described these defects are avoided by using in place of a variable mutual inductance, a variable condenser in a relatively low potential part of the filter. As will be seen later in these arrangements the tuning of the side circuits is not affected by varying this condenser. Figure 12 shows a preferred filter of this nature. As will be seen the circuit of Figure 12 is very similar to that of Figure 1, the main difference being the omission of the mutual inductance M and the provision of the condenser C In Figure 12 the references employed are those which are utilised in the following mathematical description. In Figure 12 it may be shown that the equivalent shunt inductance L3 is given by formula:

the inductance being in series with an equivalent resistance r as given by the formula:

It may also be shown that the equivalent side circuit inductance L of Figure 12 is given by the formula:

from which it will be seen that L' is independent of C. The inductance Us in shunt with Us is given by this inductance being in series with an equivalent resistance T's as given by the formula The condition for resistance balance, i. e. for T's to be equal to zero, is

l-l-wUVIG replacing L1 by L1+M1 and replacing L2 by Lz-l-M where M is the mutual inductance.

Figure 13 shows a further modification employing a combination of cross capacity coupling by condenser C and fixed mutual coupling as indicated by the bracket M. In this case the effective,

inductance in shunt with the crystal (not shown in Figure 13) consists of two inductances in parallel, one given by the inductances L1+LzM and M considered alone, andthe other due to the inductance L1, Lz-M, and the capacity C. The tuning of the side circuits is not affected in this case by adjustment of C. A fixed self-inductance may of course be used in place of M. In Figures 14 and are illustrated still further modifications (the crystal not being shown in either) wherein the efiective inductance and the shunt coupling is varied by a shunt of series condenser C. In Figure 14 the effective inductance L3 is given by L2 1 w C'L' and in Figure 15 it is given by There will now described a still further modification of the invention which has the advantage of enabling a comparatively high impedance piezo electric crystal to be used in a band pass filter suitable for use in the intermediate frequency amplifier of a radio receiver where the ratio of the pass band to the carrier frequency is small or more particularly where the carrier frequency is high, e, g. above 1,500 kc.; of reducing skirt rise even if a low impedance crystal is used; and of enabling a high gain to be obtained.

Figure 16 shows two intermediate frequency valves I and 2 coupled by a filter comprising tuning coils 3, coil 4, tuning condensers 5, a piezo electric crystal 6 cut to resonate. at the intermediate frequency, a small capacity I which may be the self-capacity of the crystal and which balances the coupling due to the coil 4 at the mid band. This filter is generally similar to the filter in Figure 1 but it may be noted that coils 8 are provided in series with the tuning condensers 5 these coils being coupled to coils 9 which are in series with resistance l0 one in the grid circuit of the valve I and the other in the anode circuit of the valve 2. H are the usual input and output circuits respectively. The resistances ll] may be variable. The coils 8 and 9 provide reaction or positive feed-back to the input and output circuits of the filter,

This filter will have an equivalent circuit similar to that shown in Figure 9 and it has already been shown in this case, that for a triple humped curve to be obtained g g, C must be greater than At high frequencies L and Q are not ordinarily very high so Le must be kept small. This is possible if the crystal is in the form of a plate vibrating in the thickness direction. Unfortunately, however, this type of crystal is very prone to spurious resonance in .close proximity to the wanted resonance, and is therefore not very satisfactory for use since the desired sharp cut-01f effects are not easily obtained. Even if a good crystal of sufiiciently low impedance were obtained the response curve would not be good in the case of a high mid-band frequency because only a low value of Q is obtainable and outside the dips the response curve would rise again very badly since the said circuits would be incapable of providing the necessary attenuation. By applying, however, (in the manner shown or in any other convenient manner) suitable reaction to the side circuits, the value of Q is effectively raised and therefore a crystal of greater impedance may be used with coils of normal value without adversely afiecting the pass band. The crystal may be of the type vibrating in the direction of its length and may be so proportioned that unwanted modes do not occur near the wanted mode, e. g. the crystal may be approximately square in the YZ plane in the case of an X-cut crystal. Further owing to the improved Q value skirt rise is considerably reduced and the effective impedance increased for the pass band thus giving better stage gain. 7

Any convenient known method of applying reaction may be used provided it does not seriously alter the tuning of the side circuits.

The detailed design of a filter as shown in Figure 16 will now be considered with the aid of Figures 171; and 17b. This will show the order of magnitude of the back coupling elements in order to obtain the required amount of reaction, and also the efiect of those coupling circuits on the design of the filter. The use of reaction afiects the reactance of the side circuits off tune, which also affects the pass band width of the filter.

The method adopted for determining the effective impedance of the side circuits Z is to find the voltage developed across Z when a given current I flows through it.

We may assume that the impedance of the back coupling coils is negligibly small compared with that of C andof R.

Considering first the side circuit connected to the anode of-a valve (Figure 17a.) when a current I flows in Z1, a voltage will be induced in the grid circuit of the valve, so that an alternating anode current is will also flow in Z1. The voltage in the gridcircuit will be due to the sumof these currents.

Thus-thetotal current flowing through the con- 'denser arm of Z and hence through the primary of which a proportion, namely V Z +R isapplied to the grid of the valve.

This voltage,

'Trz +12 R where Z the impedance constituted by Z shunted by R, i. e.

where g=the mutual conductance of the valve I Z M I rir Hence the voltage appearing across Z1 due to the current I is V and therefore the equivalent impedance of Z1 is The analysis of the case when Z1 is in the grid circuit of a valve (Figure 17b) is similar, except that the total current flowing in L must be considered to determine the voltage E on the grid of the valve.

The current in L due to I is I aC' 1 T+j wL -w) The voltage induced in series with C due to is. is

The current in L due to this voltage is T+j(wL- 7 But 1 E =jwLX the total current in L at resonance by making the term M Z 9 173" R nearly equal to unity. Final adjustments are made by varying g, R or M. In practice, it has been found most convenient to vary R.

The conditions for obtaining a given pass-band width are the same as those for the simple filter, namely that the reactance of the coupling impedance Z2 must equal the sum of the reactances of the side circuits at the given frequency.

The reactance of ZE at any frequency f ofi tune may be found from the expression 1 -1 2 A! 2,521 R, L

. 1 M 1-' k +1 1) fig Z11- 1+ 12 where Z1 and Z' are the resonance values of Z1 and. Z respectively, and I01 and k are'equal to,

f ,2) Q f0 and Q for respectively. a

In practice, it is found that the pass band width fb1 does not vary appreciably as the reaction is increased from a fairly low value to the highest value where oscillation sets in. It will simplify matters, therefore, to assume that the reaction is the maximum and Since Z is shunted by R, and we are considering frequencies relatively very close to the resonance frequency, we may neglect 70 in the denominator of the expression for ZE without much error.

Then the reactive component of ZE is If the pass band width is to be In the reactance of Z2 is ft1 c my 1 fbz and the following relation must be satisfied,

21rf 1 c f must be made larger than the value given by putting 62: in the expression for found for the simple filter.

The reaction on the side circuits may be provided in other ways known per se, e. g. by coupling the tuning condensers in the grid and anode circuit of each valve by a common resistance. Again the effective impedance on the side circuits may be raised by shunting them by a negative resistance, such as a valve having a negative anode current-anode voltage characteristic.

A practical procedure for adjusting a filter as shown in Figure 16 is as follows:

(1) Set reaction control to give zero reaction.

(2) Tune the circuits II to the crystal resonance frequency (to obtain a good output from the amplifier at that frequency).

(3) Adjust the inductance 4 or capacity 1 so that the output becomes zero at frequencies ap- Thus proximately equidistant from the mid-band frequency.

(4) Tune capacities 5 to give maximum output at crystal frequency, tuning each alternately several times in succession.

(5) Tune circuits II accurately to the crystal frequency.

(6) Decrease resistance it (increase reaction) until the desired response curve is obtained.

What we claim is:

1. An adjustable band pass filter network adapted to be connected between the output of a first electron discharge tube and the input of a second, comprising a pair of circuits each tunable to a common frequency and including a tuning condenser and an inductance, mutual coupling means between said circuits, a piezoelectric crystal having a resonance frequency equal to that to which the circuits are tuned and a shunt capacity connected between the inductances of said circuits and providing additional coupling between said circuits, means coupling one of the filter circuits to the input of the first tube, and means coupling the other of said filter circuits to the output of the second tube.

2. An adjustable band pass filter network adapted to be connected between the output of a first electron discharge tube and the input of a second, comprising a pair of circuits each tunable to a common frequency and including a tuning condenser, a main inductance and an auxiliary inductance, mutual coupling means between said circuits, a piezo-electric crystal having a resonance frequency equal to that to which the circuits are tuned and a shunt capacity connected between the main inductances of said circuits and providing additional coupling between said circuits, means coupling one of the filter circuits to the input of the first tube, and means coupling the other of said filter circuits to the output of the second tube, said last mentioned coupling means each comprising an inductance coupled to one of the auxiliary inductances of the tunable circuits.

3. An adjustable band pass filter network adapted to be connected between the output of a first electron discharge tube and the input of a second, comprising a pair of circuits each tunable to a common frequency and including a tuning condenser and a series inductance, a main inductance and a common coupling reactance completing each of said tunable circuits, a piezo-electric crystal having a resonance frequency equal to that to which the circuits are tuned and a shunt capacity connected between the main inductances of said circuits, feed-back means between the input of the first tube and the tunable circuit connected to the output thereof including an inductance coupled to the series inductance of the latter circuit, and feedback means between the output of the second tube and the tunable circuit connected to the input thereof including an inductance coupled to the series inductance of the latter circuit.

NOEL MEYER RUST. ERNEST FREDERICK GOODENOUGH. 

